Interaction: a big ship overtaking in a channel

What you do on the bridge

1. Open the gap
2. Let her pass
3. Back to the channel course

The oral questions, and what a good answer covers

The key points are in our own words. In the course, these questions come once you have done the job.

1. What is interaction between two ships, and when is it worst?Often asked

   A good answer covers:

   • Water speeds up in the gap between two hulls passing close, the pressure falls, and they are drawn towards each other
   • The pressure fields at bow and stern turn the ships too: a bow can be pushed away, then a stern drawn in, so she sheers
   • It grows with speed and falls off fast with the gap
   • It is worst in shallow water and narrow channels, and when a big ship passes a small one
   • Overtaking is the worst case, because the ships stay close for a long time

   Show me on the bridgeThe container ship coming up from astern on a line just to port of yours.

2. A big ship is overtaking you close in a channel. What do you do?Often asked

   A good answer covers:

   • Agree the overtaking by VHF, as the rules for narrow channels require, and agree which side
   • Open the gap as far as the channel safely allows, towards your own side
   • Keep a steady speed, or ease it a little to let her pass quickly; do not speed up alongside her
   • Use hand steering with a good helmsman and be ready for a sheer: counter it early with the helm
   • Watch the depth on your side of the channel, and look out for bank effect
3. What is bank effect?

   A good answer covers:

   • Close to a bank or the edge of a channel, the water between the hull and the bank speeds up and the stern is drawn towards the bank
   • The bow is pushed away from the bank, so she sheers across the channel
   • It grows with speed and with how close and how steep the bank is
   • Keep speed moderate, and carry a little helm towards the bank to hold her straight

How the lesson runs

The tutor's narration from the lesson, step by step. On the bridge, each control lights up as it is named.

1. Brief

   Inbound in the Ambrose Channel at six knots on the autopilot. A two hundred metre container ship is coming up from astern at fourteen knots.

   Her line is only sixty metres to port of yours. When she draws level, the water between you will pull you together. Don't let her get there with you that close.

2. Open the gap

   On the autopilot, alter ten degrees to starboard for a minute or so, then come back towards the channel course. Get at least a hundred metres between you and her line.

3. Let her pass

   Hold your course and speed while she goes by. Watch your heading as her bow comes level, and again as her stern passes: you'll feel her push and pull.

4. Back to the channel course

   She is clear ahead. Bring her back to two nine three, the channel course.